A linear induction accelerator is used to generate repetitively pulsed, multimegavolt, multikiloampere electron beams. Such devices are disclosed in the following: U.S. Pat. No. 2,976,444 (Kerst et al); "The Experimental Test Accelerator (ETA)", R. E. Hester et al, IEEE Transactions on Nuclear Science, Volume NS-26, No. 3, June 1979, page 4180; and "The Advanced Test Accelerator, A 50-MeV, 10-kA Induction Linac", L. Reginato, IEEE Transactions on Nuclear Science, Volume NS-30, No. 4, August 1983, page 2970.
In such a device, the accelerator includes two stages. In the first stage, high voltage pulses (typically 100-500 kV, 50-100 ns) are produced from external, water-insulated pulse forming lines. These pulses are then injected into ferrite isolated adder cavities. The effect of the cavities is to add the voltages from a large number of such cavities to produce a beam having a voltage equal to the output of the pulse forming lines times the number of cavities and a current equal to the output of the pulse forming lines.
Unfortunately, such a linear induction accelerator is too large and heavy for aerospace applications. Its irreducible weight comes primarily from the ferrite isolators and from the water dielectric in the pulse forming lines. In such a conventional accelerator, the ferrite isolators are inserted to prevent rapid shorting of the pulse forming lines output to ground. There is an inevitable inductive short to ground in such devices, but the ferrite makes the pulse impedance of the short large enough to have no substantial effect on 50-100 ns pulses.
In U.S. Pat. No. 3,171,030 (Foster et al), a "gatling gun pulse train generator" is described in which a series of pulse forming networks are charged simultaneously and discharged sequentially through a step-up transformer to produce a high voltage pulse train. Short bursts of pulses of one polarity are created. However, for a high-energy, lightweight, linear induction accelerator, a longer burst is necessary as well as a bipolar pulse. In U.S. Pat. No. 3,479,532 (Kennedy) a similar concept is disclosed, except for the use of saturable magnetic switches as opposed to vacuum/plasma switches. Still another similar apparatus is disclosed in U.S. Pat. No. 4,196,359 (North et al). In this patent, only one long pulse is generated as opposed to a pulse train.
A symmetrically charged pulse-forming circuit is disclosed in U.S. Pat. No. 4,684,820 (Valencia). This patent describes a scheme for charging a Blumlein pulse circuit in such a way that the load, such as a laser, is not disturbed by the charging process. The charging circuit produces a relatively high-voltage, fast rise time pulse and includes pulse forming networks.
A photoconductive power switch capable of producing electrical pulses of at least 10 ampere and 100 volts is disclosed in U.S. Pat. No. 4,695,733 (Pesavento). Switching speeds of one nanosecond or less can be provided. However, the ratings of such a switch are a factor of 100 to 1000 below the voltage and current requirements of a linear induction accelerator. In addition, because the switch does not open as rapidly as it closes, it would not be suitable for use in this linear induction accelerator.